selinux: use kmem_cache for ebitmap

The allocated size for each ebitmap_node is 192byte by kzalloc().
Then, ebitmap_node size is fixed, so it's possible to use only 144byte
for each object by kmem_cache_zalloc().
It can reduce some dynamic allocation size.

Signed-off-by: Junil Lee <junil0814.lee@lge.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>